A man murders his wife with a knife in their car. Nobody is around to see the crime. The man throws the wife out of the car while being very careful not to leave fingerprints. Next, he throws the knifge off a cliff where it will never be found and goes home.
An hour later, the police call him and tell him that his wife has been murdered and that he ( the husband) needs to come to the scene of the crime immediately.
As soon as he arrives he is arrested
Now... how did the police know that the husband did it?